Gelatin or other hydrophilic polymers are commonly used as binders in image recording materials such as silver-based photographic materials and ink-jet receiver materials. These products are known to be very swellable when in contact with water. The swelling property is essential in order to accomplish photographic processing chemistry or to absorb ink to generate images. However, the same property also inhibits end users from fully enjoying the product, such as handling without worry about spilling drinks or leaving fingerprints, or having to keep negatives or prints in envelopes or storage sleeves in order to avoid scratches.
The concept of applying a colloidal suspension to moist film or print material at the end of photographic processing has been disclosed in U.S. Pat. No. 2,173,480 (1939). However, since the best way to use this technology is to implement it in currently existing photofinishing equipment and laboratories, useful inventions must focus on material compositions that will best fit in with current photofinishing systems. Teachings on various methods and apparatus for applying a controlled amount of material on the silver-based photographic materials during photographic processing have been filed: U.S. Ser. No. 08/965,560 (filed Nov. 6, 1997), U.S. Pat. No. 5,905,924 and U.S. Pat. No. 5,875,370.
The temperature and residence time of photographic materials in the drying section of photofinishing trade equipment vary from 50.degree. C. to 70.degree. C. and from 30 seconds to 2.5 minutes. The actual temperature of gelatin coating during drying is much lower than the temperature set for the dryer due to the evaporation of water. In addition, it is necessary to be free of volatile organic compound (VOC) in the formulation in order to be user and environment friendly. Under these stringent requirements, it appears that an aqueous colloidal dispersion of water insoluble polymeric materials is the only appropriate system for this technology. Water soluble materials will not provide any water resistance property.
U.S. Pat. No. 2,719,791 describes the use of an aqueous dispersion of organic plastic material, which yields a water impermeable coating on drying. However, it is known that when dispersions of low Tg material (Tg&lt;25.degree. C.) are used to obtain a water resistant protective coating, the surface of the protective coating has an undesirable tacky characteristic, which generally degrades other physical properties in customers hands, such as print blocking, fingerprinting, dust attraction and high scratch propensity. When dispersions of high Tg materials (Tg&gt;25.degree. C.) are used, it is not possible to form a continuous water resistance layer on the prints under the drying condition described above. U.S. Pat. No. 2,751,315 also describes the use of aqueous dispersion of copolymer materials. It was recognized in the patent that the low Tg materials were not quite suitable and therefore higher Tg polymer in combination with a high-boiling-point organic cosolvent was used in order to form a water resistant protective coating. However, the organic solvent that is released from the formulation during drying creates an environmental concern if used in the current photofinishing laboratories with high throughput. U.S. Pat No. 2,956,877 describes the method of applying a solution that would solubilize the processing reagents from the photographic materials as well as forming a protective coating on its surface. The disadvantage of this approach is that not only can the acid groups on the polymer degrade the water resistant property of the final protective layer, but also the organic solvent required in the formulation is, again, not suitable for high volume photofinishing laboratories.
A series of patents describes the application of UV-polymerizable monomers and oligomers on imaged photographic materials followed by UV exposure to cure the formulation in order to obtain a crosslinked durable protective layer, e.g. U.S. Pat. Nos. 4,092,173, 4,171,979, 4,333,998 and 4,426,431. The major concern for this type of technology is that the use of highly toxic multi-functional monomer compounds in the formulation prevents it from being environmentally and user friendly, and the relatively short shelf life of the coating solutions.
U.S. Pat. No. 5,376,434 describes the use of at least two resins in the protective overcoat layer of a photographic print, at least one first resin having a glass transition temperature (Tg) of not less than 80.degree. C., and at least one second resin having a Tg of 0.degree. C. to 30.degree. C., wherein an arithmetic mean of the glass transition temperatures of said first resin and said second resin is 30.degree. C. to 70.degree. C. The patent teaches the use of the high Tg resin to reduce the stickiness of the overcoat due to the low Tg material.
U.S. Pat. No. 5,447,832 describes coating compositions for imaging elements comprising aqueous-based mixtures of lower Tg, film-forming polymeric particles and higher-Tg, non-film-forming polymeric particles. The film-forming particles provide continuous film formation and the non-film-forming particles comprising glassy polymers provide resistance to tackiness, blocking, ferrotyping, abrasion and scratching.
While recognizing the above-mentioned benefits of two-component aqueous dispersions cited in U.S. Pat. Nos. 5,376,434 and 5,447,832, U.S. Ser. No. 09/136,375 (filed Aug. 19, 1998; now U.S. Pat. No. 5,952,139 further disclosed preferred substituents on the high and low Tg components in two-latex formulations in order to obtain improved fingerprint resistance. Most preferred monomers are acrylonitrile, methacrylonitrile, vinylidene chloride and vinylidene fluoride.
U.S. Ser. No. 09/136,375 further describes the use of a combination of at least two aqueous colloidal dispersions of water insoluble polymeric materials for protective overcoat of silver halide photographic prints, at least one has Tg less than 25.degree. C. and at least one has Tg equal to or greater than 25.degree. C. The low Tg material comprises 20% to 95% by weight of the total material laydown, and the high Tg material comprises 5% to 80% by weight of the total material laydown. Furthermore, to provide fingerprint resistance, at least one of the materials used in the combination, regardless of its Tg, contains one or more comonomers of that invention (see formula (1) below) at 20% to 100% by weight based on the total monomers, ##STR2## wherein: X is selected from the group consisting of Cl, F or CN, and Y is each independently selected from the group consisting of H, Cl, F, CN, CF.sub.3, CH.sub.3, C.sub.2 H.sub.5, n-C.sub.3 H.sub.7, iso-C.sub.3 H.sub.7, n-C.sub.4 H.sub.9, n-C.sub.5 H.sub.11, n-C.sub.6 H.sub.13, OCH.sub.3, OC.sub.2 H.sub.5, phenyl, C.sub.6 F.sub.5, C.sub.6 Cl.sub.5, CH.sub.2 Cl, CH.sub.2 F, C.sub.2 F.sub.5, n-C.sub.3 F.sub.7, iso-C.sub.3 F.sub.7, OCF.sub.3, OC.sub.2 F.sub.5, OC.sub.3 F.sub.7, C(CF.sub.3).sub.3, CH.sub.2 (CF.sub.3), CH(CF.sub.3).sub.2, COCF.sub.3, COC.sub.2 F.sub.5, COCH.sub.3, COC.sub.2 H.sub.5.
The preferred monomers of formula (1) of this invention are acrylonitrile, methacrylonitrile, vinylidene chloride, vinylidene fluoride, vinylidene cyanide, vinyl chloride, vinyl fluoride, tetrafluoroethylene, hexafluoropropylene, perfluoropropyl vinyl ether, substituted acrylonitriles including 2-ethylacrylonitrile, 2-n-propylacrylonitrile, 2-isopropylacrylonitrile, 2-n-butylacrylonitrile, 2-n-hexylacrylonitrile, 2-trifluoromethylacrylonitrile, 2-cyanoacrylonitrile, 2-chloroacrylonitrile, 2-bromoacrylonitrile,2-ethoxyacrylonitrile, cis-3-methoxyacrylonitrile, cis-3-ethoxyacrylonitrile 2-acetoxyacrylonitrile, fumaronitrile, maleonitrile. Most preferred monomers are acrylonitrile, vinylidene chloride, and methacrylonitrile.
U.S. Ser. No. 09/354,209 of Yau et al. filed herewith, titled PROTECTING LAYER FOR IMAGE RECORDING MATERIALS describes the preferred class of materials giving superior fingerprint resistance. The glass transition temperature of the material is preferred to be lower than 30.degree. C. in order to coalesce under the mild drying conditions the image recording material experiences in photoprocessing or ink-jet printing equipment. However, during the process of coating and drying these types of latices, undesirable mobility of chemicals between image layers occurs due to the early fast film formation rate before the water is completely evaporated. The migration of chemicals within the layers can sometimes deteriorate the light fastness of image dyes.
Therefore, there is need for novel overcoat compositions for silver-based photographic and ink-jet receiver materials which can overcome the undesirable mobility of chemicals between image layers that degrades image stability to light exposure, while maintaining resistance to water, fingerprints and scratching and not adversely affecting gloss and other viewing characterisitics.